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1. Probing the Conditions for the H i-to-H ₂ Transition in the Interstellar Medium

   https://doi.org/10.3847/1538-4357/ace164  

   Park, Gyueun; Lee, Min-Young; Bialy, Shmuel; Burkhart, Blakesley; Dawson, J R; Heiles, Carl; Li, Di; Murray, Claire; Nguyen, Hiep; Hafner, Anita; et al (September 2023, The Astrophysical Journal)

   Abstract We investigate the conditions for the Hi-to-H₂transition in the solar neighborhood by analyzing Hiemission and absorption measurements toward 58 Galactic lines of sight (LOSs) along with¹²CO(1–0) (CO) and dust data. Based on the accurate column densities of the cold and warm neutral medium (CNM and WNM), we first perform a decomposition of gas into atomic and molecular phases, and show that the observed LOSs are mostly Hi-dominated. In addition, we find that the CO-dark H₂, not the optically thick Hi, is a major ingredient of the dark gas in the solar neighborhood. To examine the conditions for the formation of CO-bright molecular gas, we analyze the kinematic association between Hiand CO, and find that the CNM is kinematically more closely associated with CO than the WNM. When CNM components within CO line widths are isolated, we find the following characteristics: spin temperature < 200 K, peak optical depth > 0.1, CNM fraction of ∼0.6, andV-band dust extinction > 0.5 mag. These results suggest that CO-bright molecular gas preferentially forms in environments with high column densities where the CNM becomes colder and more abundant. Finally, we confront the observed CNM properties with the steady-state H₂formation model of Sternberg et al. and infer that the CNM must be clumpy with a small volume filling factor. Another possibility would be that missing processes in the model, such as cosmic-rays and gas dynamics, play an important role in the Hi-to-H₂transition. 

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2. The Local Group L-band Survey: The First Measurements of Localized Cold Neutral Medium Properties in the Low-metallicity Dwarf Galaxy NGC 6822

   https://doi.org/10.3847/1538-4357/ad6604  

   Pingel, Nickolas M; Chen, Hongxing; Stanimirović, Snežana; Koch, Eric W; Leroy, Adam K; Rosolowsky, Erik; Kim, Chang-Goo; Dalcanton, Julianne J; Walter, Fabian; Busch, Michael P; et al (October 2024, The Astrophysical Journal)

   Abstract Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insights into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (Hi) absorption features in NGC 6822, a low-metallicity (0.2Z_⊙) dwarf galaxy in the Local Group. These are the first unambiguous CNM detections in a low-metallicity dwarf galaxy outside the Magellanic Clouds. The Local GroupL-band Survey (LGLBS) enabled these detections, due to its high spatial (15 pc for Hiemission) and spectral (0.4 km s⁻¹) resolution. We introduce LGLBS and describe a custom pipeline for searching for Hiabsorption at high angular resolution and extracting associated Hiemission. A detailed Gaussian decomposition and radiative transfer analysis of the NGC 6822 detections reveals five CNM components, with key properties: a mean spin temperature of 32 ± 6 K, a mean CNM column density of 3.1 × 10²⁰cm⁻², and CNM mass fractions of 0.33 and 0.12 for the two sightlines. Stacking nondetections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the Hiabsorption profile encompassing the CO (2−1) emission, indicating coincident molecular gas and a depression in high-resolution Hiemission. We also analyze a nearby sightline with deep, narrow Hiself-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hαemissions suggests a close link between the colder, denser Hiphase and star formation in NGC 6822. 

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3. Revisiting the Vertical Distribution of H i Absorbing Clouds in the Solar Neighborhood. II. Constraints from a Large Catalog of 21 cm Absorption Observations at High Galactic Latitudes

   https://doi.org/10.3847/1538-4357/ad79f7  

   Rybarczyk, Daniel R; Wenger, Trey V; Stanimirović, Snežana (October 2024, The Astrophysical Journal)

   The cold neutral medium (CNM) is where neutral atomic hydrogen (H I) is converted into molecular clouds, so the structure and kinematics of the CNM are key drivers of galaxy evolution. Here we provide new constraints on the vertical distribution of the CNM using the recently developed kinematic_scaleheight software package and a large catalog of sensitive H I absorption observations. We estimate the thickness of the CNM in the solar neighborhood to be σ z ∼ 50–90 pc, assuming a Gaussian vertical distribution. This is a factor of ∼2 smaller than typically assumed, indicating that the thickness of the CNM in the solar neighborhood is similar to that found in the inner Galaxy, consistent with recent simulation results. If we consider only structures with H I optical depths τ > 0.1 or column densities N(H I) > 1019.5 cm‑2, which recent work suggests are thresholds for molecule formation, we find σ z ∼ 50 pc. Meanwhile, for structures with τ < 0.1 or column densities N(H I) < 1019.5 cm‑2, we find σ z ∼ 120 pc. These thicknesses are similar to those derived for the thin- and thick-disk molecular cloud populations traced by CO emission, possibly suggesting that cold H I and CO are well mixed. Approximately 20% of CNM structures are identified as outliers, with kinematics that are not well explained by Galactic rotation. We show that some of these CNM structures—perhaps representing intermediate-velocity clouds—are associated with the Local Bubble wall. We compare our results to recent observations and simulations, and we discuss their implications for the multiphase structure of the Milky Way's interstellar medium. 

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4. Neutral Gas Phase Distribution from H I Morphology: Phase Separation with Scattering Spectra and Variational Autoencoders

   https://doi.org/10.3847/1538-4357/ae044a  

   Lei, Minjie; Clark, S E; Morel, Rudy; Allys, E; Butsky, Iryna S; Redshaw, Caleb; Fielding, Drummond B (October 2025, The Astrophysical Journal)

   Abstract Unraveling the multiphase structure of the diffuse interstellar medium as traced by neutral hydrogen (Hi) is essential to understanding the lifecycle of the Milky Way. However, Hiphase separation is a challenging and underconstrained problem. The neutral gas phase distribution is often inferred from the spectral line structure of Hiemission. In this work, we develop a data-driven phase-separation method that extracts Hiphase structure solely from the spatial morphology of Hiemission intensity structures. We combine scattering spectra (SS) statistics with a Gaussian-mixture variational autoencoder model to (1) derive an interpretable statistical model of different Hiphases from their multiscale morphological structures, and (2) we use this model to decompose the 2D channel maps of GALFA-Hiemission in diffuse high-latitude (∣b∣ > 30°) regions over narrow velocity channels (Δv = 3 km s⁻¹) into cold neutral medium (CNM), warm neutral medium (WNM), and noise components. We integrate our CNM map over velocity channels to compare it to an existing map produced by a spectrum-based method. We find that the two maps are highly correlated, but ours recovers more spatially coherent structures at small scales. Our work illustrates and quantifies a clear physical connection between the Himorphology and Hiphase structure, and it unlocks a new avenue for improving future phase-separation techniques by making use of both Hispectral and spatial information to decompose Hiin 3D position–position–velocity space. These results are consistent with a physical picture where processes that drive Hiphase transitions also shape the morphology of Higas, imprinting a sparse, filamentary CNM that forms out of a diffuse, extended WNM. 

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5. A New Constraint on the Relative Disorder of Magnetic Fields between Neutral Interstellar Medium Phases

   https://doi.org/10.3847/1538-4357/ad5ade  

   Lei, Minjie; Clark, S E (August 2024, The Astrophysical Journal)

   Utilizing Planck polarized dust emission maps at 353 GHz and large-area maps of the neutral hydrogen (Hi) cold neutral medium (CNM) fraction (f_CNM), we investigate the relationship between dust polarization fraction (p₃₅₃) andf_CNMin the diffuse high latitude ( $\left(b\right)>30°$) sky. We find that the correlation betweenp₃₅₃andf_CNMis qualitatively distinct from thep₃₅₃–Hicolumn density (N_Hi) relationship. At low column densities (N_Hi< 4 × 10²⁰cm⁻²) wherep₃₅₃andN_Hiare uncorrelated, there is a strong positivep₃₅₃–f_CNMcorrelation. We fit thep₃₅₃–f_CNMcorrelation with data-driven models to constrain the degree of magnetic field disorder between phases along the line of sight. We argue that an increased magnetic field disorder in the warm neutral medium (WNM) relative to the CNM best explains the positivep₃₅₃–f_CNMcorrelation in diffuse regions. Modeling the CNM-associated dust column as being maximally polarized, with a polarization fractionp_CNM∼ 0.2, we find that the best-fit mean polarization fraction in the WNM-associated dust column is 0.22p_CNM. The model further suggests that a significantf_CNM-correlated fraction of the non-CNM column (an additional 18.4% of the Himass on average) is also more magnetically ordered, and we speculate that the additional column is associated with the unstable medium. Our results constitute a new large-area constraint on the average relative disorder of magnetic fields between the neutral phases of the interstellar medium, and are consistent with the physical picture of a more magnetically aligned CNM column forming out of a disordered WNM. 

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